Prior to the present invention it has long been known to sandblast paint from painted surfaces, using various types of particles, sand, or grit. The particles have been relatively dense and hard where damage to the painted surface is not an important factor. The present invention is concerned primarily with processes which will cause little damage to a metallic surface such as an airplane or a truck body, or a non-metallic surface as a circuit board.
Military and non-military specifications have been written for particles to be used (in an air-blast gun), calling for a specific gravity of about 1.2 and a Barcol hardness of 46-54, roughly equivalent to a Rockwell hardness of about 3.5. The performance specification for such particles calls for paint removal (stripping a coating of paint about 1.7 to 2.3 mils and an epoxy primer about 0.6-0.9 mil) at a rate of at least 0.15 ft..sup.2 /min. using an air-blaster capable of impelling the particles at a rate of 2.3-2.8 lbs. per second with 30 psi. See MIL-P-85891(AS), May 6, 1988, Military Specification, "Plastic Media, for Removal of Organic Coatings". Such methods of paint removal are far more acceptable environmentally than any method employing solvents. However, the particles heretofore used have either damaged the surfaces unacceptably or have not been capable of recycling because they break on impact with the painted surface; a number of polymeric particles tried have either been unable to meet the removal rate specification or, apparently because of brittleness, have become too fine for recycling through attrition. Removal rate is a function, among other variables, of the number of edges on a given particle. Polyester and other particles used in the prior art have frequently exhibited a shape with predominant flat surfaces, i.e., more or less as a flake, so that there are relatively few working edges available to strike the paint.
Both unsaturated polyesters and polymethylmethacrylate have been proposed for paint removal by blasting. See Military Specification MIL-P-85891(AS), May 6, 1988. To my knowledge, however, the particular compositions I use have not been used or suggested, and the improved results I obtain have not been seen in the past.